Systematic tuning of the emission colors and redox potential of Eu(ii)-containing cryptates by changing the N/O ratio of cryptands

Eu(ii) complexes have been widely investigated in organic light-emitting diodes (OLEDs), magnetic resonance imaging and photoredox catalysis due to their unique photophysical and electrochemical properties. To favor their further applications, it is of necessity to find a simple way to systematically regulate their emission colors and redox potentials. Herein, we have designed and synthesized a series of Eu(ii)-containing cryptates EuX2-N2O6 (N2O6 = 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, X = Br or I, similarly hereinafter), EuX2-N4O4 (N4O4 = 4,7,13,16-tetraoxa-1,10,21,24-tetraazabicyclo[8.8.8]hexacosane), and EuX2-N6O2 (N6O2 = 4,7-dioxa-1,10,13,16,21,24-hexaazabicyclo[8.8.8]hexacosane), and studied their structures and properties systematically. These Eu(ii) complexes exhibit tunable emission colors with maximum emission wavelengths in the range of 415-549 nm and photoluminescence quantum yields (PLQYs) in the range of 18-73%. On changing the N/O ratio of the cryptands, the emission colors, excited-state lifetimes, and redox potential of the corresponding Eu(ii) complexes changed linearly. The results provide a clear basis for the design of tunable, luminescent Eu(ii) complexes.